In general, an electrophotographic process using a photoreceptor is performed in the following manner. That is, the photoreceptor is charged in the dark by a charging roller in the case of, for example, contact charging method, and then exposed using LED or LD as an imagewise exposing means to selectively dissipate the charge in only the exposed areas to form an electrostatic latent image, which is rendered visible with a developer to form an image.
Fundamental properties required for the electrophotographic photoreceptors are such functions that they can be charged to a proper potential in the dark and the surface charge can be dissipated by irradiation with light.
Electrophotographic photoreceptors which are now put to practical use basically comprise a conductive support and a photosensitive layer formed thereon. However, these photoreceptors suffer from the problems that when an aluminum tube as a conductive support is subjected to cutting process by a diamond cutting tool or the like, cutting oil or powders formed by cutting remain on the support and appear as defects in formation of images after the photosensitive layer is coated on the support or when a high voltage is applied to the surface of the photoreceptor, current flows into the photoreceptors through the defects such as cutting burrs and deposited dirt or foreign matters to result in partial short-circuits. Furthermore, they appear as image defects such as dusts and fogs. Moreover, the charge generation layer formed on the conductive substrate has a thickness of about 1 μm, and is influenced by these defects to adversely affect the functions as a photoreceptor.
For inhibition of adverse effects caused by the defects on the surface of the conductive substrate, there is usually employed a method of covering the defects on the conductive substrate by providing an Alumite coat (anodized aluminum coat) on the substrate by anodizing treatment or by providing an undercoat layer using resin materials.
However, the Alumite coat has the disadvantages such as inclusion of dirt in the fine pores formed on the surface of the Alumite coat during the process of production and contamination of the surface of the Alumite coat caused at the sealing step of pores or cleaning step. Thus, even if the defects on the surface of the conductive substrate are covered, contamination of the Alumite coat per se adversely affects the photoreceptor.
It is known that resin materials such as polyethylene, polypropylene, polystyrene, acrylic resin, vinyl chloride resin, vinyl acetate resin, polyurethane resin, epoxy resin, silicone resin and polyamide resin are used for the undercoat layer. Of these resins, polyamide resins are particularly preferred.
However, in the case of an electrophotographic photoreceptor in which polyamide resin or the like is used for the undercoat layer, since the volume resistivity of the photoreceptor is about 1012-1015 Ω·cm, residual potential is accumulated in the photoreceptor unless the undercoat layer is made thin to 1 μm or less in thickness, and dusts or fogs occur in the resulting images. On the other hand, if the undercoat layer is thin, not only the defects on the conductive support cannot be covered, but also injection of holes from the substrate is accelerated during repeated use to cause considerable reduction of charging potential and decrease of light sensitivity, resulting in formation of dusts and fogs in the image to damage the image quality.
An undercoat layer is proposed which comprises a polyimide resin soluble in an organic solvent and has a thickness of 0.5 μm (e.g. Patent Document 1).
Patent Document 1: JP-A-8-30007
However, it has been found that in the case of combining the conventional charge transport agent with the undercoat layer comprising a polyimide resin and having a thin thickness of less than 1.0 μm as disclosed in Patent Document 1, the residual potential after repeated use of the photoreceptor increases, resulting in dusts and fogs in the image.
Furthermore, in the case of the electrophotographic apparatus provided with a contact charging member which applies a charging voltage by directly contacting the undercoat layer with photoreceptor, a high voltage is directly applied to the electrophotographic photoreceptor, and hence dusts and fogs are often generated.